Aedes triseriatus, the treehole mosquito, is the primary vector of La Crosse virus (LAC), the most common form of human encephalitis in the midwest. Both control and monitoring of this mosquito are exceptionally difficult. This project involves genetic and ecological research on this species, attempting to find an "Achilles heel" in vector competence. (1) Studies on genetic variability involve establishment of a formal genetics analysis of gene differences in geographic strains and clarification of relationships to a sibling species, Aedes hendersoni, a non-vector of LAC. (2) Ability to transmit LAC varies from 10-90% in different populations; genetics of this trait is under analysis and virus-refractory strains are being produced via selection. Preliminary results show that LAC is deleterious to mosquitoes in the field (affects biting behavior), leading to long term selection for resistant genotypes in the field. (3) A gregarine protozoan, Ascocystis barretti, infects the intestine of A. triseriatus. There is promising evidence that geographic strains of Ascocystis show different pathogenicity, being especially lethal against Dirofilaria-infested mosquitoes. (4) Egg diapause is controlled by photoperiod, with different levels of sensitivity in different geographical strains; genetics of this trait is under study, in the hope of using releases for field control. (5) A predatory mosquito larva, Toxorhynchites, has been released in Indiana woodlots for control of A. triseriatus. Four species of Toxorhynchites are being compared in the laboratory for biocontrol potential. (6) Dynamics of field populations is under study in experimental woodlots. Focus is on population size, longevity, vertical dispersal, female host choice and maledispersal. (7) Our WHO Reference Centre, with 40 species and 200plus strains of Aedes will continue.